CH. III.] EXTERNAL SENSATIONAL ACTIONS. 109 



mucous membrane of the nostrils, throat, and other parts. 

 It appears also, that nerves are intimately incorporated with 

 these tissues (160), for every portion of the surface which 

 the point of a needle touches, is sensitive, and the final 

 distribution of the nerves in them cannot be traced. But 

 since their structure differs from that of the muscles, we cannot 

 expect them to manifest similar sentient actions. Never- 

 theless external impressions on their nerves can proceed to 

 the brain without exciting a visible movement in the point 

 touched, — can be felt, — and can be reflected by means of 

 the internal impressions of the sensation along the principal 

 nerve and its branches, and if there be no hinderances, can 

 excite movements in the mechanical machines to which the 

 nerves are distributed, and, consequently, develope sentient 

 actions in other parts by means of the external sensation thus 

 excited on such surface (129). Thus, titillation of the mucous 

 membrane of the nostrils excites sneezing, and a convulsive 

 movement of the diaphragm and respiratory muscles, the nerves 

 of which are in natural connection with those of the nasal mucous 

 membrane : thus also many an external sensation of the cu- 

 taneous surface excites, by means of the nerves, a tremor of the 

 muscles in relation to it, which is termed a shudder. Thus also 

 a titillation of the nasal mucous membrane excites eff'usion of 

 mucus, which is a sentient action of titillation in the capillaries 

 or terminating tubuli of the minute glands (207, 172) : thus 

 also the cutaneous surface becomes inflamed, and swells from 

 the stimulus of an acrid irritant : thus also cold contracts 

 the respiratory pores by a sentient action on the minute 

 terminations of the arteries, and interrupts perspiration (168). 

 Other non-muscular tissues have either no nerves at all, or 

 very few distributed here and there in their structure. Hence 

 their sensibility is doubtful. Of this kind, according to Haller's 

 researches, are the serous membranes covering the thorax and 

 diaphragm, the pericardium, peritoneum, &c. Nevertheless, 

 experience proves that they are susceptible of certain actions of 

 external sensations; and that actions may also be excited in 

 other parts by sensations originating in their nerves. Thus 

 it is seen, that if a nerve be injured or divided which traverses 

 a tendon, or the periosteum, paralysis or convulsive movement 

 is induced in the adjoining limbs, or in those in relation with 



